We have analyzed the polyadenylated RNA expressed in cells transformed by MSV. A single LTR introduced in either a 5' or 3' relative position to the mos sequence is sufficient for enhancement or activation of the transforming potential of mos. Southern analysis of DNA's containing the integrated mos sequences DNAs confirms that tandem integrations or rearrangement of the mos and LTR sequences is not required for expression of mos or cell transformation. The enhancement properties of the proviral LTR responsible for increasing the transforming efficiency of mos have been shown to reside in a region containing the 72 base repeat sequence present the unique 3' region of the LTR. Activation of the transforming potential of c-mos is influenced by a cis-acting sequence within the 5' normal mouse DNA sequence flanking c-mos. Removal of this region from the normal mouse sequences preceding c-mos permit the upstream activation of the transforming potential of c-mos by an LTR located downstream from the oncogenic sequence.